Presently, printing employing a CTP system has been conducted in printing industries, accompanied with the digitization of printing data. A printing plate material for CTP, which is inexpensive, can be easily handled, and has printability comparable with that of a PS plate, is desired.
Particularly in recent years, a printing plate material has been sought which does not require any development employing a developer containing specific chemicals (such as alkalis, acids, and solvents), and can be applied to a conventional printing press. Known are a chemical-free type printing plate material such as a phase change type printing plate material requiring no development process, a printing plate material which can be processed with water or a neutral processing liquid comprised mainly of water, or a printing plate material capable of being developed on a printing press at initial printing stage and requiring no development process; and a printing plate material called a processless printing plate material.
A printing plate material requiring no development process or a processless printing plate material to be developed on a plate cylinder of a printing press is required to provide an exposure visualization property similarly to a conventional PS, since it is punched after imagewise exposure to form holes for mounting on the plate cylinder. After mounting a printing plate on the press, and operating a printing process, the resulting scratches and stain are desired to be removed or corrected. In this case, a printing image capable of visibility is desired to be formed in order to visually identify corrected portions. This is referred to as a printing visualization property.
A processless printing plate material is imagewise exposed employing an infrared laser with an emission wavelength of from near-infrared to infrared regions to form an image. The thermal processless printing plate material employing this method is divided into three types: an ablation type printing plate material, a development-on-press type printing plate material with a heat melt image formation layer, and a phase change type printing plate material, each described later.
Known is the following printing plate material such as a processless printing plate material having an exposure visualization property and a printing visualization property.
Examples of commonly known printing plate materials include a printing plate material having a layer containing a thermo-sensitively coloring material such as a leuco dye and a color developing agent in an image formation layer or a lipophilic oil layer containing a compound colored by a functional polymeric compound generating a sulfonic acid or a generated acid via heating (refer to Patent Documents 1 and 2), a printing plate material having a layer containing an IR-dye capable of varying optical density by exposing image formation elements (refer to Patent Document 3), and a printing plate material having a hydrophilic overcoat layer removable on a printing press, which contains at least 20% by weight of a cyanine infrared absorbing dye capable of varying optical density by light exposure (refer to Patent Document 4).
Also known is a light sensitive planographic printing plate material in which small dot reproduction, dot reproduction, and development visibility are improved by dyeing the surface of an aluminum support with a dye (refer to Patent Document 5).
However, it is seen as a problem that dyes are sublimed or scattered via laser exposure during image formation since these printing plate materials contain these dyes resulting in coloring, discoloring, or color-fading in an image formation layer via light exposure. There is also a problem such that it is difficult to avoid contamination to printing ink as well as dampening water, caused by these dyes, and a large amount of paper waste is consumed to the point where a normal printing paper sheet is obtained during development-on-press, whereby an insufficient printing visualization property results.
Since a light-to-heat conversion material and a coloring or discoloring material are dispersed in constituting layers of these printing plate materials, insufficient sensitivity and on-press developability of a printing plate material are also exhibited in the case of acquiring sufficient exposure visualization, resulting in difficulty in balancing printing suitability with exposure visualization.
(Patent Document 1) Japanese Patent O.P.I. Publication No. 2000-225780
(Patent Document 2) Japanese Patent O.P.I. Publication No. 2002-211150
(Patent Document 3) Japanese Patent O.P.I. Publication No. 11-240270
(Patent Document 4) Japanese Patent O.P.I. Publication No. 2002-205466
(Patent Document 5) Japanese Patent O.P.I. Publication No. 7-333831